Phase-Locked Loops are used in a variety of applications such as clock recovery, frequency and phase modulation, and frequency synthesizers. Voltage controlled oscillators (VCO) are central design elements of PLL's whereby the VCO produces an output frequency proportional to its input voltage. A typical drawback of a VCO is its non-linear response in output frequency to the applied input voltage. This leads to the need for a VCO having a large gain if a wide output frequency range is required. The large VCO gain also has the effect of producing a large variation in the output frequency in response to any noise in the applied input voltage, also known as phase noise. This phase noise at the VCO output is undesirable as this limits the accuracy of the output signal.
Monolithic techniques for implementing band switching utilizing varactor diodes are taught in the above-mentioned, concurrently filed patent application. Band switching techniques allow for a reduced VCO gain for a defined operating band, leading to a reduction in the VCO output phase noise.
In a band switched system, one implementation of the varactor diode band switching technique calls for five input selection sets. In response to these binary inputs, varactor diodes are digitally switched into or out of the VCO active circuitry.
A need exists in the art for a circuit for switching diodes in and out of the circuit at the proper time to reduce phase noise.
A further need exists in the art for a circuit for confining the switching of the diodes to within a fixed operating range.